1. Field of the Invention
This invention relates to a method for reading out a radiation image wherein a stimulable phosphor sheet, on which a radiation image has been stored, is exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, the emitted light is photoelectrically detected, and an image signal representing the radiation image is thereby obtained. This invention particularly relates to a method for reading out a radiation image wherein an image signal representing a radiation image is obtained which yields a reproduced visible image having a controlled sharpness and a controlled graininess.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed as desired. The processed image signal is used during the reproduction of a visible image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced as a hard copy or may be displayed on a cathode ray tube (CRT).
In the course of reading out a radiation image from a stimulable phosphor sheet, on which the radiation image has been stored, it is desired that an image signal representing the radiation image can be obtained which yields a reproduced visible image having a controlled sharpness and a controlled graininess. For this purpose, a novel method has been proposed in, for example, Japanese Unexamined Patent Publication No. 60(1985)-125056. The proposed method comprises the steps of adjusting the speeds, with which a stimulable phosphor sheet is scanned with stimulating rays in the main scanning direction and in the sub-scanning direction.
However, in cases where a rotating polygon mirror which rotates quickly is employed as a means for scanning a stimulable phosphor sheet with stimulating rays in the main scanning direction, such that a radiation image stored on the stimulable phosphor sheet can be read out quickly, it is technically difficult for the speed, with which the stimulable phosphor sheet is scanned with the stimulating rays in the main scanning direction, to be adjusted variably. Therefore, in cases where the speed, with which a stimulable phosphor sheet is scanned with stimulating rays, is to be adjusted such that an image signal representing the radiation image can be obtained which yields a reproduced visible image having controlled image quality, it is practically advantageous that the speed, with which the stimulable phosphor sheet is scanned with the stimulating rays in the sub-scanning direction, is adjusted.
However, when the speed, with which a stimulable phosphor sheet is scanned with stimulating rays in the sub-scanning direction, is changed, the resolution, with which the image information stored on the stimulable phosphor sheet is read out, also changes inevitably. In such cases, even if image signals are detected from stimulable phosphor sheets having the same size, read-out image signals will be obtained which are made up of a series of image signal components representing different numbers of picture elements in the radiation images.
In general, in apparatuses for reproducing visible radiation images from image signals, which have been obtained by reading out radiation images from stimulable phosphor sheets, and in image filing apparatuses for filing (i.e. storing) a plurality of radiation images in the form of image signals, the number of picture elements in each reproduced visible image or in each radiation image filed in the form of an image signal is set to a predetermined value at least for images having the same size. In such cases, if the number of the picture elements represented by the image signal components of an image signal detected from a stimulable phosphor sheet varies for stimulable phosphor sheets having the same size, visible radiation images cannot be reproduced or radiation images cannot be filed in the form of image signals.